The present disclosure relates generally to printed wiring board manufacturing and, more particularly, to a dummy structure and visual inspection method for printed wiring board etching.
Distributed filters are part of the toolkit for radio frequency (RF)/microwave design, and are mostly used at frequencies above the VHF (Very High Frequency) band (30 to 300 MHz). At these frequencies, the physical length of passive components is a significant fraction of the wavelength of the operating frequency, and it becomes difficult to use the conventional lumped element model. These distributed filters may be discrete, packaged devices soldered to a printed wiring board (PWB) as a surface mount component. Alternatively, to reduce cost, filters may be manufactured as part of the PWB so long as line widths and spacing of the components can be tightly controlled to guarantee filter performance. As is known in the art, printed wiring boards (PWB) are relatively difficult to fabricate. PWBs require multiple plating operations in some areas of the structure, and include etched circuits with small tolerances consisting of fine lines and/or fine gaps in other areas of the PWB. Similarly, any PWBs having etched circuits or other structures (e.g., hairpin band pass filter structures) and multiple plating steps are also relatively difficult to fabricate.
To fabricate such PWBs, prior art techniques carefully monitor fabrication and etching processes. Detail prints (i.e., fabrication drawings) define a final top layer plating thickness and also define an etch accuracy for certain top layer board features. As a result, a manufacturer (such as a fabrication or assembly house) may have to procure and produce additional duplicate PWBs in order to deliver one PWB which passes all inspections/tests and meets all of a purchaser's requirements, including critical operational requirements. PWBs which do not pass the required inspections/tests go unused. This results in wasted PWBs and higher unit cost per PWB. Requiring a PWB vendor to perform 100% measurements will also increase the cost. Moreover, even in the event a manufacturer states the PWBs meet a certain etching accuracy, it may be late into the build and integration phase of a product incorporating the PWB before such a defect is realized.